Paper Technology of MISFET

with SiO222/BaTiO333 System as a Gate InsulatorPiotr Firek and Jan Szmidt

Abstract— The properties of barium titanate (BaTiO333, BT),

such as high dielectric constant and resistivity, allow it to find

numerous applications in the field of microelectronics. In this

work silicon metal-insulator-semiconductor field effect transis-

tor (MISFET) structures with BaTiO333 thin films (containing

La222O333 admixture) acting as gate insulator were investigated.

The films were produced by means of radio frequency plasma

sputtering (RF PS) of sintered BaTiO333 + La222O333 (2% wt.) tar-

get. In the paper transfer and output I−VI−VI−V , transconductance

and output conductance characteristics of the obtained tran-

sistors are presented and discussed. Basic parameters of these

devices, such as threshold voltage (VT HVT HVT H ) are determined and

discussed.

Keywords— barium titanate, I–V characteristics, MISFET

structures, radio frequency plasma sputtering.

1. Introduction

Barium titanate (BaTiO3, BT) ceramics have been exten-

sively used in the field of electronic applications. Mul-

tilayer ceramic capacitors (MLCCs) [1], [2], embedded

capacitances in printed circuit boards [2], optical waveg-

uides [3], electrooptic modulators [4], micromechanical [5]

and humidity sensor [6] devices, positive temperature co-

efficient of resistivity thermistors [7], gas sensors [8] were

fabricated using BT. In all those applications BaTiO3 was

used in the form of either bulk material or thick layer.

BT shows ferroelectric and piezoelectric properties as well

as a high dielectric constant that make it a promising

material for potential applications in dynamic access ran-

dom memories (DRAM) [9], [10] or non-volatile memories

(NVM) [9], [11].

Thin barium titanate films for microelectronic applications

are usually either amorphous or polycrystalline and have

significantly worse electrical properties than bulk or thick-

film material. It is difficult, for example, to obtain uni-

form composition, the piezoelectric effect is weaker, and

the values of the dielectric constant are lower (typically

less then 50) [12]. On the other hand, its dielectric con-

stant is usually still much higher than that of silicon dioxide

although thin BT layers are typically plagued with higher

leakage current and lower dielectric strength.

2. Experimental Details

The fabrication process of metal-insulator-semiconductor

field effect transistor (MISFET) structures is presented

Fig. 1. MISFET fabrication process with cross-sectional view of

the structures.

61

Piotr Firek and Jan Szmidt

in Fig. 1. Its first step is thermal oxidation in order to obtain

field oxide of about 440 nm. The p-type silicon < 100 >

oriented substrate with the resistivity of 6− 8 Ωcm was

used. After cleaning processes 40 nm thick SiO2 film was

grown thermally and then a thin (approximately 80 nm) bar-

ium titanite film was deposited by means of radio frequency

plasma sputtering (RF PS) of sintered BaTiO3 + La2O3

(2% wt.) target.

A schematic diagram of the RF PS setup is shown in Fig. 2.

The BaTiO3 layer was deposited as a result of 30 min

long process (280 V self-bias voltage, argon flow rate of

Fig. 2. Schematic diagram of the setup for radio frequency

plasma sputtering deposition processes.

Fig. 3. Silicon wafer and MISFET topography.

10 ml/min and 15 mm distance between the Si substrate

and the sputtered target). Next, a photoresist mask for

etching in a buffer solution of hydrofluoric acid was pre-

pared by means of photolithography. As a last step contacts

for metallization were opened and aluminum was evapo-

rated. The described fabrication process is presented in

Fig. 2. Silicon wafer and transistor topography are shown

in Fig. 3.

3. Results and Discusions

The dielectric constant (k) of about 20 was extracted

from capacitance-voltage measurements of a MIS structure

Fig. 4. Transfer current-voltage characteristics of the fabricated

structures.

Fig. 5. Output current-voltage characteristics of the fabricated

structures.

containing BaTiO3 dielectric. The current-voltage (I−V )

characteristics of MISFETs were measured with Keithley

SMU 236/237/238. The obtained transfer and output char-

acteristics are presented in Figs. 4 and 5.

62

Technology of MISFET with SiO2/BaTiO3 System as a Gate Insulator

Threshold voltage (UT ) is one of the most important pa-

rameters of a transistor since it represents the gate voltage

at which the MISFET channel is turned on. The threshold

voltage ranged from −6 V to −8 V.

The transconductance gm and output conductance of the

structures are presented in Figs. 6 and 7, respectively.

Fig. 6. Transconductance characteristics of the fabricated struc-

tures.

Fig. 7. Output conductance characteristics of the fabricated struc-

tures.

It can be seen that the values of transconductance are rel-

atively low when compared to a typical silicon MOSFET.

Taking into consideration that these structures contain ma-

terial (BaTiO3) that is relatively novel for this type of ap-

plications and demanding from the technological point of

view, the obtained results in our opinion are very satisfy-

ing. Postprocessing (e.g., annealing) or better purity of the

films should improve the results significantly.

The dispersion of the obtained parameters (e.g., thresh-

old voltage, trans- and output conductance) may be caused

mainly by the variations of BT thickness and its composi-

tion in the area under the gate. The structure of the layer,

i.e., the grain size or the presence of amorphous phase,

the quality of the interface between the BT layer and SiO2

and the influence of plasma during fabrication may lead

to different levels of the effective charge in the dielectric

and at the aforementioned interface. As a consequence, the

transistors show different values of flat-band voltage and

threshold voltage.

4. Conclusions

The obtained BT films show good adhesion to SiO2 layers

on silicon substrate. Their relatively low dielectric con-

stant (k) (for BT) is due to their amorphous nature. High

values of the threshold voltage are a consequence of charge

presence at the SiO2/BaTiO3 interface. A better control

of the deposition process (e.g., purity) may significantly

improve the film properties. Our investigations confirm

that the RF PS method is suitable for obtaining BT layers

that may exhibit several very interesting electronic prop-

erties, especially for future IS (ion sensitive) FET struc-

tures.

References

[1] J. Zhen, Z. Yue, G. Yousong, W. Sen, L. Lingfeng, X. Zhigang,

and W. Yanbin, “Non-reducible BaTiO3-based dielectric ceramics

for Ni-MLCC synthesized by soft chemical method”, Ceram. Int.,

vol. 32, no. 4, pp. 447–450, 2006.

[2] A. Rae, M. Chu, and V. Ganine, “Barium titanate-past, present and

future”, Ceram. Trans., vol. 100, pp. 1–12, 1999.

[3] D.-G. Sun, Z. Liu, Y. Huang, S.-T. Ho, D. J. Towner, and

B. W. Wessels, “Performance simulation for ferroelectric thin-film

based waveguide electro-optic modulators”, Opt. Commun., vol. 255,

no. 4–6, pp. 319–330, 2005.

[4] P. Tang, D. J. Towner, T. Hamano, A. L. Meier, and B. W. Wessels,

“Electrooptic modulation up to 40 GHz in a barium titanate thin film

waveguide modulator”, Opt. Expr., vol. 12, no. 24, pp. 5962–5967,

2004.

[5] D. L. Polla and L. F. Francis, “Ferroelectric thin films in micro-

electromechanical systems applications”, MRS Bull., vol. 21, no. 7,

pp. 59–65, 1996.

[6] V. M. Fuenzalida, M. E. Pilleux, and I. Eisele, “Adsorbed water on

hydrothermal BaTiO3 films: work function measurements”, Vacuum,

vol. 55, no. 1, pp. 81–83, 1999.

[7] L. Affleck and C. Leach, “Microstructures of BaTiO3 based PTC

thermistors with Ca, Sr and Pb additions”, J. Eur. Ceram. Soc.,

vol. 25, no. 12, pp. 3017–3020, 2005.

[8] M. Kumar, S. Rani, M. C. Bhatnagar, and S. C. Roy, “Struc-

ture, ferroelectric and gas sensing properties of sol-gel derived

(Ba, Sr)(Ti, Zr)O3 thin films”, Mater. Chem. Phys., vol. 107, no. 2–3,

pp. 399–403, 2008.

[9] J. F. Scott, “Device physics of ferroelectric thin-film memories”,

Jap. J. Appl. Phys., vol. 38, no. 4B, pp. 2272–2274, 1999.

[10] R. Ramesh, S. Aggarwal, and O. Auciello, “Science and technol-

ogy of ferroelectric films and heterostructures for non-volatile fer-

roelectric memories”, Mater. Sci. Eng., vol. 32, no. 6, pp. 191–236,

2001.

[11] T. Kuroiwa et al., “Dielectric properties of (BaxSr1−x)TiO3 thin films

prepared by RF sputtering for dynamic random access memory ap-

plication”, Jap. J. Appl. Phys., vol. 33, no. 9B, pp. 5187–5191,

1994.

63

Piotr Firek and Jan Szmidt

[12] R. Thomas, D. C. Dube, M. N. Kamalasanan, and N. Deepak Kumar,

“Electrical properties of sol-gel processed amorphous BaTiO3 thin

films”, J. Sol-Gel Sci. Technol., vol. 16, no. 1–2, pp. 101–107, 1999.

Piotr Firek was born in Rawa

Mazowiecka, Poland, in 1977.

He received the M.Sc. degree in

microelectronics from the Fac-

ulty of Electronics and Informa-

tion Technology, Warsaw Uni-

versity of Technology (WUT),

Poland, in 2004, where he is

currently finishing a Ph.D. the-

sis. His research concentrates

on fabrication, characterization,

processing and application of thin and thick film materials

(e.g., barium titanate, boron nitride, DLC, diamond) in mi-

croelectronic devices.

e-mail: pfirek@elka.pw.edu.pl

Institute of Microelectronics and Optoelectronics

Warsaw University of Technology

Koszykowa st 75

00-662 Warsaw, Poland

Jan Szmidt received the M.Sc.

degree in electronics from the

Faculty of Electronics, War-

saw University of Technology

(WUT), Poland, in 1976. From

the same university, he received

the Ph.D. and D.Sc. degrees in

1984 and 1995, respectively. In

1999, he became an Associate

Professor. In 2002 he became

an Associate Dean for Develop-

ment of the Faculty of Electronics and Information Tech-

nology, Warsaw University of Technology. Since 2006 he

has been the Head of Electronic Materials and Microsys-

tem Technology Division, Institute of Microelectronics and

Optoelectronics, WUT and since 2008 – the Dean of the

Faculty of Electronics and Information Technology WUT.

His research interests concentrate on technology and char-

acterization of thin films for electronics, especially for mi-

croelectronics and nanoelectronics, as well as on their ap-

plication in microelectronic and nanoelectronic structures.

e-mail: j.szmidt@elka.pw.edu.pl

Institute of Microelectronics and Optoelectronics

Warsaw University of Technology

Koszykowa st 75

00-662 Warsaw, Poland

64